From: Dan Williams Subject: Re: [PATCH 0/9] add ext4 per-inode DAX flag Date: Thu, 7 Sep 2017 13:54:45 -0700 Message-ID: References: <20170905223541.20594-1-ross.zwisler@linux.intel.com> <20170906170754.GB17663@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Cc: Eric Sandeen , Lukas Czerner , Andrew Morton , "linux-kernel@vger.kernel.org" , "Darrick J. Wong" , "Theodore Ts'o" , Andreas Dilger , Christoph Hellwig , Dave Chinner , Jan Kara , linux-ext4 , "linux-nvdimm@lists.01.org" , linux-xfs@vger.kernel.org To: Ross Zwisler Return-path: Received: from mail-oi0-f48.google.com ([209.85.218.48]:36321 "EHLO mail-oi0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752375AbdIGUyr (ORCPT ); Thu, 7 Sep 2017 16:54:47 -0400 Received: by mail-oi0-f48.google.com with SMTP id x190so4850854oix.3 for ; Thu, 07 Sep 2017 13:54:47 -0700 (PDT) In-Reply-To: <20170906170754.GB17663@linux.intel.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, Sep 6, 2017 at 10:07 AM, Ross Zwisler wrote: > On Tue, Sep 05, 2017 at 09:12:35PM -0500, Eric Sandeen wrote: >> On 9/5/17 5:35 PM, Ross Zwisler wrote: >> > The original intent of this series was to add a per-inode DAX flag to ext4 >> > so that it would be consistent with XFS. In my travels I found and fixed >> > several related issues in both ext4 and XFS. >> >> Hi Ross - >> >> hch had a lot of reasons to nuke the dax flag from orbit, and we just >> /disabled/ it in xfs due to its habit of crashing the kernel... > > Ah, sorry, I wasn't CC'd on those threads and missed them. For any interested > bystanders: > > https://www.spinics.net/lists/linux-ext4/msg57840.html > https://www.spinics.net/lists/linux-xfs/msg09831.html > https://www.spinics.net/lists/linux-xfs/msg10124.html > >> so a couple questions: >> >> 1) does this series pass hch's "test the per-inode DAX flag" fstest? > > Nope, it has the exact same problems as the XFS per-inode DAX flag. > >> 2) do we have an agreement that we need this flag at all, or is this >> just a parity item because xfs has^whad a per-inode flag? > > It was for parity, and because it allows admins finer grained control over > their system. Basically all things discussed in response to Lukas's original > patch in the first link above. I think it's more than parity. When pmem is slower than page cache it is actively harmful to have DAX enabled globally for a filesystem. So, not only should we push for per-inode DAX control, we should also push to deprecate the mount option. I agree with Christoph that we should try to automatically and transparently enable DAX where it makes sense, but we also need a finer-grained mechanism than a mount flag to force the behavior one way or the other.